Electron Paramagnetic Resonance Saturation Studies of P700 exp + Reaction Center Chlorophyll in Plant Photosynthesis

摘要

Electron paramagnetic resonance (EPR) power saturation and saturation recovery methods have been used to determine the spin-lattice, T sub 1 and spin-spin, T sub 2 , relaxation times of P700 exp + chlorophyll in photosystem I reaction centers of plant chloroplasts as a function of temperature. T sub 1 varied between 9 x 10 exp -4 seconds at 10 exp 0 K and 2 x 10 exp -4 seconds at 80 exp 0 K. T sub 2 varied between 1.2 x 10 exp -7 seconds at 7 exp 0 K and 4 x 10 exp -8 seconds at 40 exp 0 K, corresponding to spin packet linewidths of 0.05 and 0.15 mT respectively. Incubation of D144 fragments with 10 mM ascorbate did not affect P700 exp + spin-lattice relaxation. Ferricyanide was used to poise broken chloroplasts at a redox potential of 460 mV. Upon illumination at 40 exp 0 K these samples exhibited a free radical signal which had a spin-lattice relaxation time of 8 x 10 exp -5 seconds. The short relaxation time is caused by the presence of the ferricyanide. The spin-spin interaction between P700 exp + and both iron sulfur center A and plastocyanin was also examined. Neither reduced center A nor oxidized plastocyanin exhibited any effect on P700 exp + relaxation times. It was calculated that the minimum separation distance for the P700-center A pair and separation for the P700-plastocyanin pair is greater than or equal to 30 A. A nonsaturating power level for the P700 exp + signal at 25 exp 0 K was determined by plotting a power saturation curve. A ratio of 4.8 +- 0.4 was achieved. This indicates that a spin state above the spin = 1/2 state of center A may be significantly populated at low temperature. If so, the entire EPR signal of center A may not be observable under the present conditions and the P700 to center A ration may be lower than measured here. Considering all of the above, the earlier model of a direct 1:1 relationship between P700 and center A can no longer be supported. A more satisfying model would include electron carriers A sub 1 , A sub 2 , and center B. (ERA citation 05:012552)